1. Field of Invention
This invention relates to semiconductor light emitting devices including a filter.
2. Description of Related Art
Semiconductor light emitting devices such as light emitting diodes (LEDs) are among the most efficient light sources currently available. Material systems currently of interest in the manufacture of high brightness LEDs capable of operation across the visible spectrum include group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials; and binary, ternary, and quaternary alloys of gallium, aluminum, indium, arsenic, and phosphorus. Often III-nitride devices are epitaxially grown on sapphire, silicon carbide, or III-nitride substrates and III-phosphide devices are epitaxially grown on gallium arsenide by metal organic chemical vapor deposition (MOCVD) molecular beam epitaxy (MBE) or other epitaxial techniques. Often, an n-type region is deposited on the substrate, then an active region is deposited on the n-type region, then a p-type region is deposited on the active region. The order of the layers may be reversed such that the p-type region is adjacent to the substrate.
The color of light emitted from a semiconductor light emitting device chip such as a light emitting diode may be altered by placing a wavelength-converting material in the path of the light exiting the chip. The wavelength-converting material may be, for example, a phosphor. Phosphors are luminescent materials that can absorb an excitation energy (usually radiation energy) and store this energy for a period of time. The stored energy is then emitted as radiation of a different energy than the initial excitation energy. For example, “down-conversion” refers to a situation where the emitted radiation has less quantum energy than the initial excitation radiation. The energy wavelength effectively increases, shifting the color of the light towards red.
A common method of making a light emitting device that emits white light is to combine a phosphor such as Y3Al5O12:Ce3+ that emits yellow light with a blue LED chip that emits blue light. The combination of yellow phosphor-converted light and unconverted blue light leaking through the phosphor layer appears white. The color characteristics of the combined light are controlled by selecting only LEDs that emit blue light of a particular wavelength, and by varying the thickness of the phosphor layer to control the amount of leakage of blue light and the amount of phosphor conversion. This approach is inefficient in that large numbers of LEDs which emit blue light at a wavelength outside the desired range are unusable, and results in large variations in the correlated color temperature (CCT) of the light since it is difficult to precisely control the amount of blue leakage and phosphor conversion. The CCT of phosphor converted LEDs sold today may vary from 5500K to 8500K. Discernable color differences are dependent on the CCT of the combined light. At 6500K, differences as small as 300K are apparent to the viewer. The large variation in CCT between parts is unacceptable for many applications.